Field of the Invention
The present invention relates to a controller for a machine tool or the like, the controller having an abnormality detection function for CPUs used therein.
Description of the Related Art
A central processing unit (CPU) may possibly undergo thermal runaway, which may cause a fire in the worst case, if its junction temperature is exceeded. Therefore, it is necessary to mount a temperature sensor in the vicinity of the CPU or monitor the current consumption of a CPU power supply so that the CPU can be stopped in case of thermal runaway.
FIG. 3 is a diagram illustrating a conventional technique for detecting thermal runaway of a CPU in a controller in advance.
A controller 1 comprises a central processing unit (CPU) 2. A current monitoring circuit 3 is mounted in the vicinity of a power supply circuit of the CPU 2 for detecting thermal runaway of a CPU in advance. This technique requires the use of a high-precision current detection IC and its peripheral circuits, as well as a space for mounting the current monitoring circuit.
FIG. 4 is a diagram illustrating a conventional CPU abnormality detection technique disclosed in Japanese Patent Application Laid-Open No. 2-254330.
According to this technique, a plurality of ICs (ICs 5 and 6) in an electronic device 4 are provided with a plurality of temperature sensors, individually. Specifically, the IC 5 is provided with temperature sensors 7 and 8, and the IC 6 is provided with temperature sensors 9 and 10. A protection device 20 for protecting the ICs of the electronic device 4 comprises a CPU 21, memory (RAM/ROM) 22, A/D converter 23 for A/D conversion of signals, and multiplexer 24 for input signal switching.
IC temperature data detected by the temperature sensors 7 to 10 are processed by the protection device 20. If temperature differences (between the temperature sensors 7 and 8 and between the temperature sensors 9 and 10) exceed a predetermined value, the temperature sensors are determined to be out of order. If the temperatures of all the temperature sensors exceed a predetermined value when the temperature differences are not higher than the predetermined value, the temperatures are determined to be abnormal. According to this technique, an element to be temperature-controlled requires the use of the temperature sensors and the protection device for monitoring them.
In a method for monitoring heat/current consumption, a threshold should be set at a rather high value in consideration of an instantaneous current. In some cases, therefore, a system may be stopped after the CPU undergoes thermal runaway so that the CPU and its peripheral devices may be damaged. If the temperature sensors are used for monitoring purpose, moreover, normal operation cannot be performed in case of their thermal runaway, so that the system cannot be stopped.